An optical device which controls the transmittance (or reflectance) of incident light needs a high contrast ratio and a high efficiency of light utilization.
One well-known optical device for controlling transmittance of light with voltage application is the liquid crystal panel. A liquid crystal panel includes a pair of substrates and a liquid crystal layer which is provided between these substrates. In a liquid crystal panel, liquid crystal molecules within the liquid crystal layer undergo changes in their alignment depending on the level of voltage being applied across the liquid crystal layer, which results in changes in the transmittance of light entering the liquid crystal panel. Liquid crystal panels have a very high contrast ratio, and therefore are widely used in display devices.
However, many liquid crystal panels are of the type that employs polarizers; therefore, half or more of the light which is utilized for displaying is absorbed at the polarizers. This results in a low efficiency of light utilization. Accordingly, development of optical devices which do not require polarizers has been under way in the recent years.
Patent Documents 1 and 2 disclose an optical device including a suspension layer which contains polymer flakes. In this optical device, the polymer flakes can be rotated with application of an electric field to the suspension layer, whereby the optical characteristics of the suspension layer are altered.
Patent Document 3 discloses a transflective display including a suspension layer which contains reflective particles. FIG. 9 shows a suspended particle device 800 which is used in the transflective display that is disclosed in Patent Document 3.
As shown in FIG. 9, the suspended particle device 800 includes a suspension layer 830 containing reflective particles 832 such as metal particles. The suspension layer 830 is provided between an electrically insulative substrate 810 and a transparent plate 820. On the sides of the electrically insulative substrate 810 and the transparent plate 820 that face the suspension layer 830, ITO layers 811 and 821 are formed as electrodes.
Spacers 860a and 860b are provided between the electrically insulative substrate 810 and the transparent plate 820. On the respective side faces of the spacers 860a and 860b, ITO layers 870a and 870b are formed as electrodes. Passivation layers 880a, 880b, 880c, and 880d of SiO2 electrically isolate the ITO layers 870a and 870b from the ITO layers 811 and 821 that are on the electrically insulative substrate 810 and the transparent plate 820.
When a voltage is applied between the ITO layers 811 and 821, the reflective particles 832 will align so that their longitudinal direction runs parallel to the direction of the electric field, i.e., so that their longitudinal direction is perpendicular to the substrate plane. As a result of this, the transmittance of the suspension layer 830 increases.
On the other hand, when a voltage is applied between the ITO layers 870a and 870b formed on the side faces of the spacers 860a and 860b, the reflective particles 832 will align so that their longitudinal direction runs parallel to the direction of the electric field, i.e., so that their longitudinal direction is parallel to the substrate plane. As a result of this, the reflectance of the suspension layer 830 increases (and its transmittance decreases). Thus, the suspended particle device 800 experiences changes in transmittance and reflectance as the direction of the applied field is changed.
The optical devices of Patent Documents 1 and 2 and the suspended particle device 800 of Patent Document 3 mentioned above do not require polarizers, and therefore can attain a higher efficiency of light utilization than that of a liquid crystal panel.